Without limiting the scope of the present invention, its background will be described in relation to systems for reducing agglomeration of injected powder and, in particular, to systems, lances, nozzles, and methods for powder injection resulting in reduced agglomeration, as an example.
With the introduction of the first national standards for mercury pollution from power plants in 2011, many facilities will turn to activated carbon injection (ACI) to meet the regulatory requirements. ACI is a mature technology that is widely available and proven for achieving mercury removal to the required levels. ACI involves the pneumatic conveyance of a powdered activated carbon (PAC) or other type of powdered sorbent from a storage silo into the process gas of a power plant. Once introduced to the process gas, the sorbent adsorbs mercury. The sorbent and associated mercury is separated from the process gas by a particulate removal device resulting in lower concentrations of mercury in the gas.
Most development work for this methodology has focused on controlling and modifying the PAC or other powdered sorbents to maximize their potential reaction with mercury. The result of these efforts has been to introduce oxidizing components to the sorbent as well as minimize particle size and pore diffusion resistance to accelerate the kinetic reaction to match system constraints.
Commercial systems are readily available for powder flow transport and delivery and have been adapted for ACI systems. These systems include a powder storage vessel with some means of fluidization, a flow path to a feed hopper that uses various combinations of valves and screws to convey powder to an eductor, and transport lines that convey transport air and powder to injection lances, as further discussed below with reference to FIGS. 1A1-1C2. The entire system must operate continuously without plugging or clogging to introduce powder to the flue gas. The delivery of the powder from the injection lances is critical to the resulting reactions in the process gas. ACI installations at existing power plants are limited by the given process framework and therefore locations where the ACI system can be installed and (therefore where the powder can be injected) is limited.
This often constrains the available contact time for PAC to adsorb mercury (as defined by the time the PAC enters the gas environment in the duct system to the time it is removed by the particulate control device). In addition to this constraint, process gases are generally high volume flows thereby having low concentrations of contaminants, especially in the case for mercury. With limited time and low concentration of the target contaminant to be removed, it is critical to maximize the dispersion (and therefore reactive surface area) and the particle distribution of the sorbent in the process gas to in turn maximize the contaminant removal potential.
As used herein, “dispersion” will be referred to as the degree of agglomeration, or how well the actual average particle size matches that of the primary particle size. Highly dispersed powders are those in which there is little to no agglomeration and the actual average particle size is equivalent to the primary particle size. Low dispersion would conversely mean that there is a large degree of agglomeration and the actual average particle size is much larger than that of the primary particle size. Distribution of the particles herein will refer to the degree that particles are separated from each other and their location in the process gas. High dispersion conditions will have particles that are well separated and fill the process gas volume thereby having good interaction with the process gas. Low dispersion conditions may have streamlining where particles are in closer proximity to each other and occupy only a portion of the available volume.
Fine powders have an inherent cohesive tendency and form agglomerates. Therefore, when agglomerates form, the average particle size of the powder being introduced into the process gas will have a larger size than the primary particle size originally produced. Larger particle size leads to less available reactive external surface area and less particle gas interaction. This works against high levels of contaminant removal from process gas. This is especially so in the case of mercury removal from flue gas where mercury removal is mass transfer limited. Despite the efforts to improve the PAC reactivity, if its surface area is not readily accessible, mercury removal will be limited.
This theory was demonstrated using an in-line particle size measurement tool that monitored particle size in the flue gas with increasing injection rates. The tendency of PAC to agglomerate increased with increased particle feed rate. As PAC agglomerates, much of its surface area gets blocked off and is no longer exposed directly to the process gas. These measurements explain the phenomena of the mercury removal performance plateau exhibited in many ACI systems despite increasing injection rates. Therefore, if agglomeration can be eliminated and particles reduced back to their primary particle size, the linear trend of increasing mercury removal with increasing injection rates could be maintained.
Several styles of lances are commonplace in ACI systems currently employed. Referring to FIGS. 1A1-1C2, several prior art lances are discussed. The main style of lance employed is a lance 10 having a tubular body 12 and one end 14 that connects with a supply of pneumatically powered powder, such as ACI (FIGS. 1A1-1A2). Lance 10 may have a second end 16 that is substantially square or flat and having an opening 18 where ACI flows outward. Another configuration includes a lance 20 having a tubular body 22 and one end 24 that connects with a powdered ACI (FIGS. 1B1-1B2). Lance 20 may also include a second end 26 that may be angled and having an opening 28 where ACI flows outward.
While these lance designs have low tendency to plug, they also do a poor job of dispersing and distributing the powder particles. Increasing the number of lances in a cross section and staggering their injection lengths can improve this distribution. More lances, however, comes with the drawback of larger pressure drop in the lines which can lead to sedimentation and clogging or necessitate a larger flow or pressure air supply. Another commonly applied design tries to improve particle distribution by using an injection lance 30 having a tubular body 32 and one end 34 that connects with a powdered ACI (FIGS. 1C1-1C2). Lance 30 may also include several holes 34 along the vertical length of body 32. Tubular body may have a closed end 36. While this design allows for better distribution of powder across the duct cross-section and less lances, it is prone to clogging. These traditional lances, without a mechanism to de-agglomerate powder particles, lack the ability to finely disperse the powder particles which limits the upper level of possible contaminant removal despite how well of a distribution can be achieved.
In another method in previous use, a system to re-mill PAC just prior to injection into the flue gas may be used. This would remove agglomerates formed during storage and transportation. However, during conveyance and injection in the ACI system, particles will have a chance to re-form agglomerates thereby limiting the potential of the powder to react and capture mercury. In addition, if the applied lance design used in conjunction with the milling system does not distribute these particles well in the flue gas cross section, mercury removal will again be limited.
Another method to introduce additives to contaminant-containing gases includes compressing a gas, contacting the additive with the compressed gas, mixing the additives and gas, and delivering the mixture to contaminant-containing gas stream delivery system of a manifold and many conduits, or lances. While this system claims to improve distribution of the additives, it does not include methods to improve particle distribution.
Another method for ACI known to those skilled in the art is a typical pneumatic injection system that injects particles into a gas duct that contains a static mixer. The design is intended to improve powder distribution in the duct itself and requires little to no maintenance. While this method improves on particle distribution, again, there are no means to disperse the particles to their primary particle size and increase the available reactive surface area. Additionally, integrating the static mixer requires replacing a portion of the duct while the power plant is not operating incurring high cost and inconvenience.
Another system is an integrated mercury control system that includes a sorbent injection subsystem wherein the system adapted to disperse sorbent particles within the flue gas stream at a defined rate for the sorbent particles to adsorb gaseous pollutants. As part of this subsystem, injection lances were comprised of various conduits. These conduits consist of a first end where the sorbent material is fed into and the product travels down the length of the conduit to a second end where the sorbent particles are released. The varying vertical lengths allow for greater distribution of the particles. While the disclosure claims that the amount of clumping of particles is reduced by the straight flow, low-pressure conduits with no curves or angles, there are no means to disperse already formed clumps or agglomerates.